Although fragile X syndrome (FXS) is a single-gene (i.e., FMR1) disorder associated with a characteristic behavioral phenotype, there is considerable within-syndrome variability in the severity of affectedness and the profile of neurocognitive impairments and co-morbid conditions displayed. About 25-30% of people with FXS also meet diagnostic criteria for autism (AUT), with the remainder displaying autistic-like behavior below the diagnostic threshold. In this project, we focus on understanding within-syndrome variability in language development in relation to AUT status among boys with FXS. We propose to use a well-established paradigm (i.e., novel word learning) to study the ways in which children with FXS (with and without AUT) use social- affective cues to learn new words (Specific Aim 1); how such learning processes impact later language outcomes (Specific Aim 2); and how differences in AUT status and symptoms and anxiety, as well as in the environment and the FMR1 gene, contribute to within-syndrome variation in these processes (Specific Aim 3). Four groups of boys will participate, with the groups matched on nonverbal mental age: FXS only; co-morbid FXS and AUT; idiopathic AUT; and typically developing. At Time 1, each boy will participate in three word- learning studies, each focused on the use of a different type of social-affective cue. Assessments of AUT symptoms, anxiety, cognition, and language will also be conducted. Maternal synchrony, a dyadic interaction strategy known to support language development, and FMRP levels will also be measured. 1.5 years later, each boy's language will again be assessed. ANOVA, multiple linear regression, and path analysis will be used to compare the performance of the groups in the word learning tasks, to examine the relationship between Time 1 word learning and later language, and to examine the concurrent relationships among word learning, autism symptom domains, anxiety, maternal synchrony, and FMR1 variation. The project will be conducted at the Waisman Center (Wisconsin) and the M.I.N.D. Institute (CA). The project is unique in its inclusion of these three atypical groups and its focus on the processes underlying language problems.